Extend the JSON writer to support writing comments.

Add a data type for handling reader parameters.

src/json.m:
	Add predicates for injecting comments into a JSON writer stream.

	Add a structure that holds all the reader parameters -- this is in
	preparation for upcoming changes that will add further parameters.

	Rename one of the fields of the json.reader/1 type.

src/json.writer.m:
	Implement output of comments.

src/json.json_parser.m:
src/json.json_lexer.m:
	Conform to the above field name change.

tests/test_json.m:
	Conform to the above change.